It is known in the art to utilize heat shrink or shrink fit tool assemblies in industrial applications. The prior designs have utilized unitary chucks having an inner diameter slightly smaller than the exterior diameter of the shank of the cutting tool or other work piece. During use, the tool chuck is heated, typically by means of induction heating, causing the chuck to expand a sufficient amount to allow the tool shank to be inserted within the chuck. The tool is then allowed to cool during which the thermal contraction of the chuck exerts a uniform pressure on the tool shank, allowing the chuck to shrink down around the tool shank to securely lock or hold the shank in place. It has long been recognized that these types of heat shrink tool assemblies provide an improvement in accuracy and rigidity over standard milling chucks or collet chucks.
Referring now to FIG. 6, a conventional shrink fit tool holder is shown generally at 10 for detachably retaining a rotary cutting tool 14. In general, the shrink fit tool holder 10 comprises a shank or body 12 that includes a tool holder portion 12a, a flange member 12b, a tapered outer surface 12c that generally corresponds to a tapered bore of a spindle (not shown), and a central bore or aperture 12d for accommodating the cutting tool 14.
The central aperture 12d is formed to be slightly less in diameter than a shank portion 14a of the cutting tool 14. This amount depends on the nominal size and the required torque transmission capacity. In order to insert the cutting tool 14 within the tool holder 10, the tool holder portion 12a of the tool holder 10 is externally heated, for example, to a temperature of approximately 650° F. Due to the thermal expansion characteristics of the tool holder 10, the application of heat to the tool holder 10 causes the tool holder 10, and in particular, the tool holder portion 12a of the body 12 to expand resulting in the central aperture 12d enlarging or increasing in diameter a sufficient distance to allow the tool shank 14a to be inserted within the central aperture 12d. When the tool shank 14a is inserted a sufficient distance within the central aperture 12d, the external application of heat is discontinued and the tool holder 10 is allowed to cool back to ambient temperature, wherein thermal contraction causes the aperture 12d to contract and form a rigid bond between the tool holder 10 and the shank portion 14a of the cutting tool 14. As such, the cutting tool 14 is rigidly maintained within the tool holder 10 in a concentric fashion for high tolerance machining applications. To remove the cutting tool 14, heat is again applied to the tool holder 10 causing the tool holder 10, and in particular, the tool holder portion 12a to expand outwardly. As the tool holder 10 expands, the tool shank 14a can be withdrawn from within the central aperture 12d of the tool body 12.
However, the use of shrink fit tool holders for machining of high strength materials, such as in the aerospace industry, has some limitations. The maximum grip is proportional to the interference fit levels, but a maximum designed interference is limited by the temperature required to remove the tool from the tool holder. Therefore, the maximum grip is limited.
Some end mills with high helix angles may be pulled out from the tool holder during severe operating conditions, thus scrapping costly components. One solution is to use Weldon shank holders that provide high torque, but are not suitable for high speed applications.
Accordingly, there is a need for a heat shrink tool assembly that provides an increased torque transmission capability of the assembly.